Games that run on a Freemium model will have two modes:
Play for free. You can play as long as you like without paying, but some amount of content will be unavailable, and you might have to deal with annoying advertisements.
Upgrade to a paid account. This gives you full access to the features that weren't available for free, but you have to shell out some dough for the privilege (almost always a monthly fee).
Unlike games that use Microtransactions, Freemium games have a binary distinction between those who pay and those who do not: you're either Free, or you're Premium. This tends to limit the Bribing Your Way to Victory effect—if you can only pay a fixed amount, you can't out-pay the other people who are paying. That's not to say that the two payment methods can't be combined—there are plenty of Freemium games that also feature Microtransactions.
Offering a 30-Day Free Trial of the Premium version is commonplace, but if the Free version itself is a 30-Day Free Trial, it's not this trope.
If the game's marketing plays up features in the paid version of the game while advertising it as "Free", you probably have an Allegedly Free Game on your hands.
Of course, this model isn't limited to games. It's a common business model all over the internet. Even on this very wiki, making a modest donation to the site will temporarily remove the advertisements from the forums. *nudge nudge*
Compare and contrast Shareware.
See also: Allegedly Free Game
"Remember folks, this segment is for Platinum viewers only! So if your next trip has an in-flight movie instead of an in-flight safari, why don't you run along and count how many frequent flyer miles you need to upgrade to an aisle seat? beat Are they gone? Good."
Club Penguin accounts are upgraded via monthly subscription.
RuneScape members are treated to a vastly larger game world with more servers, no ads, loads of exclusive content, weekly game updates, and a bunch of other stuff in exchange for a monthly fee. Sister site FunOrb runs on a similar system.
AdventureQuest requires a one-time fee of $20 in order to upgrade your account to a "Guardian", unlocking additional content.
Marvel's Super Hero Squad Online has a free play mode, which gives you some starting heroes and lets you play the Mission of the Day, and requires a monthly fee for additional heroes and missions.
Gem Craft does this. $5 gets you a handful of additional skills and some new battle settings, some of which are necessary to unlock all of the levels.
Stick Arena Ballistick. Anyone can play the basic game. Buying a Lab Pass for $5.95 (1 month), $29.95 (6 months) or 57.95 (1 year) will allow you access to new arenas, weapons, pets and spinners.
Ragnarok Online has a Free server and a Premium server. The Premium server gives more exp, higher drop rate, and items which can not be obtained on the free server. In some regions, the Free servers actually have more items... except they are all Purposefully Overpowered items intended to break the game's balance. Of course, these items are only available for renting with real cash for a short time - that's right, you can't buy them, so if you want to stay on the competitive game you're paying a fee either way.
Many Sony Online Entertainment games have been switched to a 'Free to Play, Your Way!' model. This model offers 3 tiers of play: Free, Premium and Legendary. Free players can play almost all content up to level Cap barring Downloadable Content but they have less item and bank slots and a lower money limit, as well as only 2 character slots. More can be bought and a purchase of at least $5.00 upgrades a player to Premium status with access to more item slots and money than Free players but less than Legendary players. Both tiers must additionally purchase any DLC they wish to play. Legendary requires a (albeit small) monthly subscription fee, opening up all the game's original (Pay to Play) features. Missing a payment downgrades a player to Premium until the next payment. Everquest, Everquest 2, DC Universe Online and Vanguard: Saga Of Heroes use this model.
Quake Live: Although you can play for free, by paying for the Premium account, you get extra maps, modes, and the ability to create your own dedicated servers, among other things.
Team Fortress 2 went Free-To-Play when the Uber Update was released in Summer 2011. Free (or Limited) accounts only get to store up to 50 items in their Backpack, can only receive items rather than being able to Trade or give gifts, have limited Crafting blueprints and can't get Rare (such as the Halloween Items) or Cosmetic items. Otherwise, they can get all of the regular weapons, and all game modes and maps are available to both Free and Premium accounts note Some private servers either ban free players or tag "F2P" to their names, however. Worth mentioning is that the only requirement for "premium"/Retail is buying any item from the in-game store, which can be as cheap as $0.50; in fact, the only difference between a premium account gained from buying the game itself as opposed to something from the in-game store is a Proof of Purchase hat.
DarkOrbit features two kinds of currencies: credits, which may be obtained generating and selling minerals (for free; it just requires you to be clickittyclickittypatient); and Uridium, which are obtained mainly paying a monthly fee. You can find both types of currency roaming randomly through the space and completing missions, but Uridium are much rarer, and, of course, Uridium buy the coolest things.note To clarify: in the shop, all items have a fixed price, in credits or Uridium. You can still auction any item for credits, but you need lots of luck or lots of money. Or both. Or logging in at ungodly hours when no one's awake (it kind of helps that Dark Orbit servers are country-specific).
Sryth has a lot of content available for free. Buying a subscription (9.95 USD for 3 months, or 19.95 USD for 1 whole year) grants access to even more content: The ability to log in regardless of server load, no ads, more character slots (4 instead of 2), more adventures, more events, more locations to visit, a way to learn all skills and powers instead of just some of them, Grand Residences, Multiplayer scenarios… See this page for an incomplete list of the things subscribers get.
Dungeons & Dragons Online was one of the first MMORPGs to essentially combine Allegedly Free Game and Freemium, into three distinct payment models: Free to Play, Premium, and VIP. The first, which is like many Allegedly Free Game's and requires earning or buying points to unlock content - using Microtransactions. In this model, the player technically plays for free, but continuing to play the game this way requires a lot of grinding, and dealing with multiple limitations (which can mostly be bypassed through the store) in order to progress past the first 8 to 12 levels. The second two are more along the lines of Freemium, with the only exception being that the Premium level requires some form of payment - whether that be purchasing an access to a pack of dungeon's, subscribing for a month, or even buying any amount of points. This mode removes a lot of the more draconian limitations placed on free players, but still requires purchasing quest packs with points. Finally, the old tried-and-true subscription model works exactly how it does in most other MMORPGs. Namely, unlimited access to content, save for some of the newly introduced races and classes. Subscribers also get a 500 point allowance per month to spend in the store for various convenience items.
City of Heroes became this in 2011. There's 3 tiers: Free, Premium, and VIP. Free is, of course, someone who's never paid a dime. They don't have access to Chat Channels, the in-game Auction House, Mission Architect, end-game system and even certain Classes, and also don't have Posting access on the forums. They do, however, have access to 99% of the content of the game, levels 1-50, no purchases of any kind required. Premium is anyone who's EVER paid anything who may or may not have the limits described above (for instance, one can buy access to the Market or Mission Architect, and once one has bought enough stuff, they're automatically granted certain privileges such as Forum posting and the previously-locked Classes), and VIP are monthly subscribers who get everything listed above, plus certain Online-Store items for free, as well as a free stipend of points to use in the Online Store and other perks.
ARMA 2: Free is a free-to-play counterpart to ARMA 2 released two years after the retail game, advertised as free of microtransactions and allowing Free players to play alongside or against players who'd bought the game. Unlike ARMA 2 however, the free version does not include the official campaigns, has toned-down graphics quality compared to the paid versions, and does not support the use of addons — so no custom guns, vehicles, characters, DayZ, etc. As a result, they can only play on ARMA 2: Free servers or on ARMA 2 servers that are not running custom player-made addons, unless they were to buy ARMA 2 or Operation Arrowhead.
Draw Something has a free version with ads and a pay version ($.99) without ads.
Serious Sam HD: The Second Encounter has gone this route - competitive multiplayer is now free to play, with the actual singleplayer/co-op game available as the "Campaign DLC".
In addition to items and experience boosts obtained by spending Gold, bought with real money, any Tribes: Ascend player who has ever spent any amount of money on the game gets VIP status which grants a permanent 50% EXP bonus; this stacks multiplicatively with the bought experience booster packs. VIP will also grant you access to premium servers available only to paying players, if they ever come online that is.
Blackout Rugby has a lot of features for its Premium members, such as a 2D field to watch the games in (instead of the standard text crawl). Also, Premium adds plenty of club customization options. You can even rename your players.
Around the time of the Cataclysm expansion, World of Warcraft went free-to-play up to level 20.
Furcadia is possibly the Ur Example; since 1997, you could get wings for your characters, and entirely new species for a certain price. In their kickstarter campaign, they mentioned they've been using a Freemium model before it was a word.
World of Tanks gives 50% bonus on all earned experience and game money to users with paid "premium" accounts. While it is entirely possible to get any of the top tanks for free, a few of them are too expensive to repair and require either a premium account, lighning-fast reflexes and strategy or another tank to earn money for two. T-50-2 is one such example — as a Tier 5 light scout, that frequently has to face top-tier opponents, it gets killed very often. The game also uses microtransactions, allowing to purchase: more room in the hangar (useful, if you want more than 5 tanks), better shells (sometimes deadly, but usually not very useful) and rare tank models (which are no better than regular, but allow to avoid Level Grinding).
PlanetSide 1 had the Reserves program, which ran for a full year in ~2006. Anyone could set up an account and play, but free players were limited to Battle Rank 6 and Command Rank 1, which severely limited what they could do - forcing them into Crippling Overspecialization or making them a Master of None. Battle Frame Robotics were totally out of reach for Reserve players. Reserve players who upgraded to the standard $14.99 per month subscription fee had those restrictions removed.
Planetside 2 is totally free-to-play (aside from cosmetic items), but people who pay $14.99 per month subscription fee for "Membership" status gain $5.00 worth of in-game currency a month, 50% more XP, 2x passive certification gain, and higher priority in server/continent queues. Members also gain access to additional sales and early access to cosmetic items.
The Day Nine Daily, a web show for Starcraft II, has an "optional subscribers" program where for $5 a month viewers get to play games with Day himself, among other little bonuses.
The web radio service Pandora is free to use, but upgrading to Pandora One provides such perks as unlimited listening time, pause/rewind/skip options, no ads, and such.
Hulu offers free video streaming for popular TV shows and movies. Signing up for "Hulu Plus" expands the available video library significantly, as well as letting you watch (some) streams on a Playstation 3. Unlike other freemium services, paying for Hulu Plus does not remove the advertising.
The online music service Spotify has three tiers: Open (completely free, with advertisements and time restrictions), Unlimited (advertisements and time restrictions are removed), and Premium (no ads, no time restrictions, plus additional perks).
Megavideo (RIP), a video hosting website, used to charge a membership fee for some of its features.
Some file download sites offer superior service to users who purchase premium memberships. These include faster download speed, no time between downloads, no download limits and the ability to download multiple files at once. Examples:
Scribd: Getting a subscription allows you to download as many documents as you like for a specific period of time.
deviantART removes ads for paying members. Successful artists may have premium memberships gifted to them — for instance, a fan or friend pays for theirs.
Premium DA members also have tons of other privileges, such as the ability to post multiple images at once, create polls, and see who's visited their profile page. However, unlike some of the other examples here, DA members with free accounts still enjoy many benefits and aren't excluded from most activities and functions.
LiveJournal's paid accounts allow for 30 userpics, get more space for photos and own an exclusive LJ email address, among other perks. They used to be able to allow for comment-editing as well until 2012, when this feature was extended to free accounts.
A running gag on 4chan / Facebook / Ponychan is to post an image that says "Image requires a 4chan/Facebook/Ponychan GOLD account". People will then post what an awesome picture it was, prompting someone to eventually ask "How do I get GOLD acct.?!" and thus outing themselves as a noob.
4chan actually plays this trope straight since late 2012, offering a "4chan Pass" that removes captchas required to post for $20 a year.
Modeling and photography networking site Model Mayhem offers a free account with a basic front page and a fifteen-image portfolio. For more pictures and fancier page options, one can upgrade to one of several "VIP" account levels.
A number of news websites operate this way; the websites of The New York Times, The New Republic, and The Economist all allow you to view a certain number of articles (e.g. 10 for the Times) each month, but if you want to read any articles beyond that you have to subscribe (to either the print edition or to an online-only edition) (they can tell when you've exceeded the limit through IP and device tracking). The news/culture/analysis aggregator blog The Dish by AndrewSullivan operates on the "freemium" model as well (in this case, allowing subscribers to "read on" to more in-depth analysis more often) and actively evangelizes for the system as the best way forward for digital news media.
Many mobile-device apps use the "free, or pay to get rid of the ad displays" model.
The Escapist has something called "The Publisher's Club." If you pay the membership fee, you get to watch the videos in higher quality, you get access to a special forum for the members, and instead of the usual titles given to forum members depending on how many posts they've got under their belt, you can create your own.